There is substantial demand for piezoelectric ultrasonic transducers (UTs), such as those formed by piezoelectric films, sandwiched between two electrode layers. In a wide variety of contexts, the ability to propagate ultrasonic waves in a medium, and/or detect waves thus propagated, is highly useful, for example, in ultrasonic non-destructive testing (NDT) and structural health monitoring (SHM) of materials, components or structures. In some applications, there is a particular need: to use broad frequency bandwidth UTs; to perform ultrasonic generation or detection at elevated temperatures; or to conform the piezoelectric transducer to components or structures that have complex shapes, such as curved surfaces like pipes. For example, there is a need to monitor the thickness of pipes in a power plant that is subjected to high temperatures, wear, corrosion or erosion. If a high accuracy of the thickness measure is desired, a broad bandwidth UT is advantageous. Typically it is highly desirable that UTs operate in the broadband frequency regime in which their −6 dB bandwidth exceeds 30%, to emit/detect ultrasonic pulses that have only a few ring-down cycles, allowing for high precision thickness measurement and also the defect location, if any. Therefore there are high demands for UTs that can perform ultrasonic measurements efficiently and accurately (a) on curved surfaces, (b) at high temperatures with capability to sustain thermal cycles from low temperature such as −80° C. to elevated temperatures such as 200° C., 500° C., 800° C. or 1000° C.; and (c) a broad frequency bandwidth. It is also desirable to have flexible UTs which have the above (a), (b) and (c) features.
While broad frequency bandwidth can be provided by mechanically damping the UT, e.g. with a backing between a top electrode of the UT and its air interface (i.e. on the opposite side of the inspected surface). Backings attenuate the ultrasonic (mechanical) energy, as is known in the art (see, for example U.S. Pat. No. 3,376,438 to Colbert [1], U.S. Pat. No. 3,989,965 to Smith et al. [2], and G. Kossoff, “The effects of backing and matching on the performance of piezoelectric ceramic transducers”, IEEE Trans. on Sonics and Ultrasonics, vol. SU-13, pp. 20-30, March 1966. [3]). When the piezoelectric element is excited by the electrical signal, the generated ultrasonic wave transmitted into the backing material will be attenuated by the mechanisms of absorption and/or scattering. It suppresses multiple reflected echoes inside the piezoelectric element and thus it will have the broadband frequency characteristics. However, the use of backings in accordance with the prior art, introduce other problems. Backing materials often use epoxies as the host materials in which metal or ceramic powders are filled in order to increase the acoustic impedance of the backing and match with that of the UTs. This backing makes the previous arts bulky, heavy, and not flexible and difficult to be used to evaluate materials, components and structures with complex surfaces. In addition, epoxies cannot sustain a temperature more than several hundred degrees Celsius so that it is not suitable for high temperature applications. It should also be noted that for measurements at high temperatures, thermal cycles may happen. Because of the large difference in thermal expansion coefficients and thermal conductivity between the epoxy and the thin metallic electrode of the UT, epoxy-based backings tend to detach from the electrode after several thermal cycles. Such detachment causes an abrupt failure that degrades the broadband frequency characteristics of the UTs.
Another approach to achieving broad frequency bandwidth UTs is to insert a matching layer between the UT and inspected surface, the matching layer having the proper acoustic impedance according to those of the piezoelectric transducer material and the medium, as known in the art (see, for example [3], U.S. Pat. No. 2,430,013 to Hansell [4], or U.S. Pat. No. 4,016,530 to Goll [5]). When one layer with quarter wavelength thickness and proper acoustic impedance, i.e. square root of the product of the acoustic impedance of piezoelectric material and that of the target to be monitored, are inserted between the UT and the medium, the bandwidth can be increased. However, it is relatively difficult to obtain such a proper acoustic impedance material. Also, the increase of bandwidth using this technique is limited. Multiple layers could be used to accomplish the acoustic impedance conditions, although it can cause further loss and the design becomes more complicated. Furthermore, acoustic impedance will change when the temperature changes, and each material has a different acoustic impedance dependence with temperature. Therefore, it is at least difficult to provide high quality impedance matching with a variety of ultrasonic media, for operation across a wide temperature range, which is often required for NDT and SHM applications.
According to the teachings of U.S. Pat. No. 4,751,013 to Kaarmann et al. [6], porosity is introduced into piezoelectric films with a view to reducing shear wave excitation at the transducer edge and to match the acoustic impedance of the UT to that of the substrate to be inspected, so that more ultrasonic energy can be transmitted from the UT to the substrate. There is no information relating porosity to frequency bandwidth, or temperature of operation, and no evidence that bonding during thermal cycle or flexibility would be provided. Furthermore, the disclosed method of fabricating porous piezoelectric films was by mixing piezoelectric powder, binder, and polymer in the form of small particles which were fired out during calcination process. Since the sizes of pearl polymers were between 10 μm to 40 μm which are large, high ultrasonic attenuation and strong scattering at high ultrasonic operation frequency are expected.
U.S. Pat. No. 6,111,339 to Ohya et al. [7] teaches manufacture of porous piezoelectric sheets. There is no information relating the porosity to the frequency bandwidth, or operating temperature, and no evidence that bonding during thermal cycle or flexibility would be provided. Furthermore, the disclosed method of fabricating porous piezoelectric films was by mixing piezoelectric powder, binder, and combustible powder such as poly methyl methacrylate which will burn out during heating process. The pore sizes in this method were between 5 and 25 μm which are still large and result in high ultrasonic attenuation due to strong scattering at high ultrasonic operation frequency.
U.S. Pat. No. 5,585,136 to Sekimori et al. [8] teaches a particular fabrication technology, sol-gel technique, to produce piezoelectric films for ultrasonic transducers. The invention reported is related to how to reduce the porosity in order to fabricate dense piezoelectric films. There is no information relating the porosity to the frequency bandwidth, or temperature of operation, and no evidence that bonding during thermal cycle or flexibility would be provided. Also US patent application US2008/182128 to Boy et al. [9] teaches a method to produce low porosity piezoelectric films with high piezoelectric constant by multiple impregnation of the porous film with sol-gel piezoelectric precursor solution. This method is laborious and results in piezoelectric transducers that are narrow band and do not have the high temperature capabilities.
It is also known to provide high porosity UTs. For example, U.S. Pat. No. 5,958,815 to Loebmann et al. teaches a method of producing a particular piezoelectric film for a transducer designed for coupling to a gaseous medium. As noted in the field of that invention, the notably different acoustic impedance of solids and gasses make conventional ultrasonic sensors and actuators made of dense ceramic and ceramic-polymer composites, of limited use in coupling to gaseous media. Loebmann therefore only advocates use of porous UTs for coupling to coupling with gaseous media. The prior art shows a bias for dense UTs when coupling with solids or liquids. It will be noted that their UTs are about 80% porous, making them ill suited for coupling to solid or liquid media.
Accordingly there is a need for broad frequency bandwidth ultrasonic transducers capable of operating at high temperatures such as 200° C., 500° C., 800° C. or 1000° C., preferably without requiring a backing.